1. Technical Field
The present invention relates to a digital controlled oscillator used for a semiconductor integrated circuit, a frequency synthesizer including the same, a control method for the same, and a radio communication apparatus using such a frequency synthesizer.
2. Background Art
With semiconductors being finer and faster, a frequency synthesizer using a digital controlled oscillator (DCO) circuit controlled with a digital value, not an analog value, has been studied (see Japanese Laid-Open Patent Publication No. 2002-33660 and U.S. Pat. No. 6,734,741, for example).
FIG. 19 shows an exemplary conventional DCO circuit. As shown in FIG. 19, the conventional DCO circuit includes an inductor 201 and a variable capacitance section 202. The inductor 201 and the variable capacitance section 202 constitute an LC tank circuit, which outputs a signal having a frequency corresponding to the inductance of the inductor 201 and the capacitance of the variable capacitance section 202.
The variable capacitance section 202 has capacitor arrays 203 to 205 each composed of a plurality of varactors. The varactors are variable capacitors whose capacitance value changes between a low capacitive state and a high capacitive state larger in capacitance value than the low capacitive state. Therefore, by changing the capacitance values of the varactors from the low capacitive state to the high capacitive state with a control signal supplied to each capacitor array, the capacitance value of the variable capacitance section 202 changes.
The amount of change in frequency for each unit control signal, that is, the amount of change in frequency observed when the capacitance value of one variable capacitor is switched from its low capacitive state to its high capacitive state differs among the capacitor arrays. For example, the frequency changes by 3 MHz in the capacitor array 203, changes by 320 KHz in the capacitor 204, and changes by 10 KHz in the capacitor array 205. The capacitor arrays therefore cover different oscillation frequency ranges from one another as shown in FIG. 20. The oscillation frequency ranges covered by the capacitor arrays overlap each other, so as to permit the oscillation frequency of the DCO circuit to change in fine steps and also change widely.
However, the conventional digital controlled oscillator has the following problem. In wide-range modulation, to suppress increase in quantization noise, it is necessary to not only permit wide-range frequency variation but also reduce the minimum frequency change to as small as possible and yet keep the change amount constant.
To suppress increase in quantization noise, the frequency change for each unit control signal must be kept constant. In wide-range modulation, therefore, it is necessary to increase the number of control bits while keeping the minimum frequency change amount, that is, the minimum capacitance change amount of the variable capacitance section constant.
The conventional DCO circuit, having the variable capacitance section composed of three capacitor arrays, permits frequency variation over a considerably wide range. However, the changes in oscillation frequency at the operation of the respective capacitor arrays are independent of one another. For wide-range modulation, therefore, it is necessary to increase the number of varactors included in the capacitor array 205 to secure a wide range within which the frequency can be changed linearly. For example, to change the oscillation frequency in 64 steps linearly, at least 63 varactors will be necessary in the capacitor array 205. This causes a problem of increasing the area occupied by the variable capacitance section and thus the area occupied by the DCO circuit. Also, 64 control lines will be necessary to control the varactors in the capacitor array 205 individually. This will increase the area occupied by such control lines, and with the area increase, the parasitic capacitance will further increase.
As described above, if the number of control bits is increased while the unit capacitance is kept small, the occupation areas of the capacitors and the control lines will increase. With the area increase, the parasitic capacitance will also increase. The increase of the parasitic capacitance may cause a problem of narrowing the variable range of the oscillation frequency.